Overview: The goal of this proposal is to identify the cellular and molecular mechanisms that regulate interactions between the tracheobronchial epithelial tissue-specific stem cell (TSC) and its niche. Dysregulation of the TSC self-renewal and differentiation program has been suggested for many upper airway diseases like COPD, asthma, and CF. My preliminary data indicate that mouse tracheal TSCs generate their own niche that is promitotic in nature and modification of this niche is necessary for differentiation. This modification is accomplished by environmental cues derived from differentiated tracheal cells. Based on this preliminary data, I will determine 1) the role of these TSCs in tissue repair, and 2) the signaling pathways involved in maintenance of the proliferative niche. Significance of the study: This study will advance the field of lung TSC by addressing 2 fundamental paradigms: 1) TSC participation in repair of the damaged epithelium, and 2) signaling pathways that allow differentiation of transit amplifying cells (TAC). Finally, this study provides an important modification of the "seed and soil" paradigm for cell-based therapy by raising the possibility that introduction of pure stem/niche cells to an injured organ may result in unrestrained expansion of the mitotic cohort due to disease-associated depletion of the differentiated cells that modify the niche. Innovation: Development of a rim-clone assay allows generation of large number of TAC (8x10^3) from a single TSC. This method will permit analysis of signaling mechanisms using TSC from genetically-modified mice, through cell-mixing studies and using pharmacological approaches. Thus, I will have the ability to evaluate TSC proliferation and differentiation under controlled conditions for the first time.